U.S. Pat. No. 7,157,887 describes direct amplitude modulation for switch mode power supplies. The switch mode power supply comprises a first branch of a first inductive element; a first switching element and a second switching element connected in series, and a second branch of a second inductive element; a fourth switching element and a third switching element connected in series. Both branches are coupled to a power source in parallel. A load is connected between the braches, in between the connected first and second switching element and the third and fourth switching element. The switch mode power supply operates in a plurality of consecutive time periods, wherein more than two of said switching elements are closed, i.e. at least one in each branch. For example in such a period the first branch delivers power to the load from the conductive element, whereas the inductive element in the second branch is simultaneously charged from the power source.
Due to the simultaneous operation of both branches, also called direct amplitude modulation (DIRAM), a controllable power boost is achieved, which enable increasing the output voltage. Such output voltage has a square-wave shape of both polarities, which is applicable to rectification. After rectification a smooth DC voltage is achieved. However, when the switch mode power supply has to deliver a specific voltage, the output voltage cannot be controlled accurately.